1. FIELD OF THE INVENTION
The present invention relates to an electrophotographic printer comprising plural light emitting devices, and a method for processing an image signal in an electrophotographic printer comprising plural light emitting devices, more particularly, to an electrophotographic printer comprising plural light emitting devices each of which has a light output opening elongated in a sub scan direction perpendicular to a main scan direction, and a method for processing an image signal therein.
2. DESCRIPTION OF THE RELATED ART
In a conventional printer utilizing the electrophotographic process, after a photoconductive drum is electrified uniformly, it is exposed to light of image data so as to form an electrostatic latent image thereon. As light source means for exposing the photoconductive drum to light thereof, there have been used aligned light emitting diodes (referred to as LEDs hereinafter) of an linear LED array which are driven according to image data so as to project emitting light through each light output openings thereof onto the photoconductive drum, or light source system for selectively projecting light emitted from a light source such as a halogen lamp, a fluorescent lamp onto the photoconductive drum utilizing a linear liquid crystal shutter. It is to be noted that the light emitted through each light output opening from the light source is projected onto the photoconductive drum through an optical system such as a cell fox lens, a rod lens. The above electrostatic latent image is developed with toner so as to form a visible toner image on the photoconductive drum, and the visible toner image is transferred onto a piece of printing paper. Thereafter, the visible toner image is fixed thereon, resulting in a printed image of a document on the printing paper.
In the conventional light sources of this type, such as the linear LED array, the aforementioned light source system comprising the linear liquid crystal shutter, plural light output openings for outputting light of each emitting device such as an LED or each shutter device are aligned in a longitudinal direction of the printing line or in a main scan direction, and each light output opening has a shape of almost square.
FIG. 1 shows an example of a linear LED array 2a comprising plural light output openings 3a of LEDs which are aligned in the main scan direction at a pitch of 80 .mu.ms in a high density of 300 dots/inch, wherein each light output opening 3a has a longitudinal length of 80 .mu.ms in the subscan direction corresponding to the y-axis direction which is perpendicular to the main scan direction corresponding to the x-axis direction, and a width of 70 .mu.ms in the main scan direction. It is to be noted that the width of each light output opening 3a in the main scan direction is smaller than the length thereof in the subscan direction since plural LEDs are arrayed.
If each light output opening 3a is elongated in the subscan direction, the light amount of light emitted from each LED can be increased without decreasing the image density in the main scan direction. Therefore, when the aforementioned linear LED array 3a is used, the power consumption thereof can be decreased, and also the life of each LED thereof can be extended.
Further, when there is used the aforementioned light source system comprising the linear liquid crystal display shutter, the illumination intensity of the light source such as a halogen lamp, a fluorescent lamp can be set at a relatively smaller value, resulting in improvement of the illumination efficiency.
However, only when each light output opening 3a is elongated in the subscan direction, the quality of the printed image such as the contrast thereof, the definition thereof deteriorates.